1. Field of the Invention
The present invention relates to a plane light source unit, and more particularly to a plane light source unit which may be advantageously used as a backlighting means for a liquid crystal display device or the like.
2. Description of the Related Art
Conventionally, a backlighting means for a liquid crystal display device or the like generally has such a construction that a linear lamp is located at a focus of a reflector having a parabolic section while a milk-white diffusing plate is located above the lamp. Various improvements were made to optimize the configuration of the reflector, to adjust the diffusion coefficient of the diffusing plate.
Special constructions have also been provided which employ such a combination of a linear lamp and a light guide, one side end of which is adjacent to the lamp, and the configuration of the light guide is simulated by approximation to a point source and worked into a curved surface so as to emit the light in a particular direction, or the thickness of the light guide is varied along the direction of light, or else a lenticular is used wherein the prism angle is varied in accordance with the distance from the lamp, or otherwise such configurations as described above are combined suitably.
Recently, plane light source units have been used as liquid crystal display units. However, the display device, which uses a conventional plane light source unit to obtain a high quality display, requires a thick plane light source unit. In particular, a plane light source unit for a large display with a size of 10 to 12 inches requires the conventional unit having a thickness of 20 to 30 mm. Therefore, the conventional unit is impossible to be used as a thin plane light source unit.
Various plane light source devices of edge-light types have been proposed. Each of these devices has a transparent light guide comprising a plate-shape transparent material such as an acrylic resin. Incident light from one end surface of the transparent light guide is then emitted from the upper or lower surface of the light guide. However, such a large liquid crystal display device with a size of 10 to 12 inches results in a display where luminance is decreased according to the distance from the light source or display with uneven luminance, so that a display of high quality cannot often be obtained.
In order to solve this problem, a light guide having a thickness which varies according to the distance from a lamp, or a member which changes light path geometrically, is provided. However, such a member having a specific shape requires a precise work to form, and thus it becomes expensive to prepare. Furthermore, this member does utilize only a small portion of the incident light to the member.
Recently, Japanese Patent Application Laid-open No. (Tokukai-Hei) 1-245220 disclosed a plane light source unit of an edge-light type. This plane light source unit has a light guide with a layer of a light diffusing material which is applied or adhered on the surface thereof opposite to a light emitting surface or on a light diffusing surface of a provided member. The density of the amount of the light diffusing material applied increases as the distance from the portion of light incidence increases. Japanese Patent Application Laid-open No. (Tokukai-Hei) 1-107406 disclosed a plane illuminating device which can uniformly illuminate the whole surface of a light diffusing plate. This device has a plurality of piled transparent plates, each of which has a different pattern of fine dots of light diffusing material thereon.
Since a non-light-transmissive inorganic material (e.g., white pigment such a titanium oxide or barium sulfate) is used, a loss of light occurs to lower the luminance of the emitted light.
Oe, one of the inventors of the present application, disclosed in U.S. Pat. No. 4,729,068 a light diffusing layer provided on the light guide through an intermediate layer having an intermediate characteristics of those of the light guide and the diffusing layer, and a light reflecting film (a light regulation member) to evenly illuminate the whole surface of the diffusing device.
The inventors of the present application proposed a plane light source unit which is an edge-light type, as described in Japanese Patent Application Laid-open Nos. (Tokukai-Hei) 1-244490 and 1-252933. This unit has a lens-like surface or a satin finished surface provided on at least one of a light emitting surface of a light guide and the opposite surface thereof, and a light regulation member having a light reflecting pattern which reflects most of the light near the light source but transmits almost all light near its central part, and a light diffusing plate provided on the emitting surface of the light guide.
The light diffusing device and the plane light source unit using the light regulation member show excellent effects regarding the uniformity in luminance of the emitted light. The device and the unit, however, cannot utilize the light which is reflected by the light regulating member, and accordingly the luminance of the emitted light lowers near the minimum value of the luminance before the regulation.
To provide a plane light source unit with an uniform and high luminance, the inventors of the present application proposed in U.S. patent application Ser. No. 07/849,365 filed on May 5, 1992 a plane light source unit which is characterized in that at least one of the light emitting plane and the opposite surface of the transparent light guide has a directional light emitting function which emits the incident light from the light incident surface of the transparent light guide in an oblique direction to the incident light and has a regulation function which makes the luminance of the light uniform over the light emitting surface in the whole surface thereof. This plane light source unit has been designed to emit light as uniformly as possible in all directions. However, it is desired to concentrate emitted light in a particular direction depending upon application of a plane light source.
For example, in the case of application to a liquid cryatal color TV set for personal use or a liquid crystal display for a personal computer having a comparatively small viewing angle, it is required to emit the light only in a particular direction and make the amount of emitted light as uniform as possible over an entire emitting surface.
The inventors of the present application disclosed, in U.S. Pat. No. 5,126,882 and Japanese Patent Application Laid-open Nos. (Tokukai-Hei) 2-17 and 2-84618, a plane light source unit comprising a first element having a lens-like surface or a satin finishied surface provided on at least one of a light emitting surface of a light guide and the opposite surface thereof, thereof, and a second element having a large number of prism units formed on the emitting surface which emitted the light from the emitting surface of the first element in a prescribed direction. This plane light source unit can easily form the light concentrated in a direction to be observed by a user. However, this unit is unsatisfactory in the uniformity and value of luminance of the emitted light.